First non-repeating character using one traversal of string | Set 2

Given a string, find the first non-repeating character in it. For example, if the input string is “GeeksforGeeks”, then output should be ‘f’ and if input string is “GeeksQuiz”, then output should be ‘G’.

We have discussed two solutions in Given a string, find its first non-repeating character . In this post, a further optimized solution (over method 2 of previous post) is discussed. The idea is to optimize space. Instead of using a pair to store count and index, we use single element that stores index if an element appears once, else stores a negative value.

Implementation:

C++

// CPP program to find first non-repeating
// character using 1D array and one traversal.
#include <bits/stdc++.h>
using namespace std;
#define NO_OF_CHARS 256

/* The function returns index of the first
non-repeating character in a string. If
all characters are repeating then
returns INT_MAX */
int firstNonRepeating(char* str)
{

    // Initialize all characters as
    // absent.
    int arr[NO_OF_CHARS];
    for (int i = 0; i < NO_OF_CHARS; i++)
        arr[i] = -1;

    // After below loop, the value of
    // arr[x] is going to be index of
    // of x if x appears only once. Else
    // the value is going to be either
    // -1 or -2.
    for (int i = 0; str[i]; i++) {
        if (arr[str[i]] == -1)
            arr[str[i]] = i;
        else
            arr[str[i]] = -2;
    }

    int res = INT_MAX;
    for (int i = 0; i < NO_OF_CHARS; i++)

        // If this character occurs only
        // once and appears before the
        // current result, then update the
        // result
        if (arr[i] >= 0)
            res = min(res, arr[i]);

    return res;
}

/* Driver program to test above function */
int main()
{
    char str[] = "geeksforgeeks";
    int index = firstNonRepeating(str);
    if (index == INT_MAX)
        cout <<"Either all characters are "
               "repeating or string is empty";
    else
        cout <<"First non-repeating character"
               " is "<< str[index];
    return 0;
}

// This code is contributed by shivanisinghss210

C

// CPP program to find first non-repeating
// character using 1D array and one traversal.
#include <limits.h>
#include <stdio.h>
#include <math.h>
#define NO_OF_CHARS 256

/* The function returns index of the first
non-repeating character in a string. If
all characters are repeating then
returns INT_MAX */
int firstNonRepeating(char* str)
{
    // Initialize all characters as
    // absent.
    int arr[NO_OF_CHARS];
    for (int i = 0; i < NO_OF_CHARS; i++)
        arr[i] = -1;

    // After below loop, the value of
    // arr[x] is going to be index of
    // of x if x appears only once. Else
    // the value is going to be either
    // -1 or -2.
    for (int i = 0; str[i]; i++) {
        if (arr[str[i]] == -1)
            arr[str[i]] = i;
        else
            arr[str[i]] = -2;
    }

    int res = INT_MAX;
    for (int i = 0; i < NO_OF_CHARS; i++)

        // If this character occurs only
        // once and appears before the
        // current result, then update the
        // result
        if (arr[i] >= 0)
            res = min(res, arr[i]);

    return res;
}

/* Driver program to test above function */
int main()
{
    char str[] = "geeksforgeeks";
    int index = firstNonRepeating(str);
    if (index == INT_MAX)
        printf("Either all characters are "
               "repeating or string is empty");
    else
        printf("First non-repeating character"
               " is %c", str[index]);
    return 0;
}

Java

// Java program to find first
// non-repeating character
// using 1D array and one
// traversal.
import java.io.*;
import java.util.*;
import java.lang.*;

class GFG
{
/* The function returns index
of the first non-repeating
character in a string. If
all characters are repeating
then returns INT_MAX */
static int firstNonRepeating(String str)
{
    int NO_OF_CHARS = 256;

    // Initialize all characters
    // as absent.
    int arr[] = new int[NO_OF_CHARS];
    for (int i = 0;
             i < NO_OF_CHARS; i++)
        arr[i] = -1;

    // After below loop, the
    // value of arr[x] is going
    // to be index of x if x
    // appears only once. Else
    // the value is going to be
    // either -1 or -2.
    for (int i = 0;
             i < str.length(); i++)
    {
        if (arr[str.charAt(i)] == -1)
            arr[str.charAt(i)] = i;
        else
            arr[str.charAt(i)] = -2;
    }

    int res = Integer.MAX_VALUE;
    for (int i = 0; i < NO_OF_CHARS; i++)

        // If this character occurs
        // only once and appears before
        // the current result, then
        // update the result
        if (arr[i] >= 0)
            res = Math.min(res, arr[i]);

    return res;
}

// Driver Code
public static void main(String args[])
{
    String str = "geeksforgeeks";

    int index = firstNonRepeating(str);
    if (index == Integer.MAX_VALUE)
        System.out.print("Either all characters are " +
                       "repeating or string is empty");
    else
        System.out.print("First non-repeating character"+
                             " is " + str.charAt(index));
}
}

Python3

'''
Python3 implementation to find non repeating character using
1D array and one traversal'''
import math as mt

NO_OF_CHARS = 256

'''
The function returns index of the first
non-repeating character in a string. If
all characters are repeating then
returns INT_MAX '''

def firstNonRepeating(string):
    #initialize all character as absent

    arr=[-1 for i in range(NO_OF_CHARS)]

    '''
    After below loop, the value of
    arr[x] is going to be index of
    of x if x appears only once. Else
    the value is going to be either
    -1 or -2.'''

    for i in range(len(string)):
        if arr[ord(string[i])]==-1:
            arr[ord(string[i])]=i
        else:
            arr[ord(string[i])]=-2
    res=10**18

    for i in range(NO_OF_CHARS):
        '''
        If this character occurs only
        once and appears before the
        current result, then update the
        result'''
        if arr[i]>=0:
            res=min(res,arr[i])
    return res

#Driver program to test above function

string="geeksforgeeks"

index=firstNonRepeating(string)

if index==10**18:
    print("Either all characters are repeating or string is empty")
else:
    print("First non-repeating character is",string[index])

#this code is contributed by Mohit Kumar 29

C#

// C# program to find first
// non-repeating character
// using 1D array and one
// traversal.
using System;

class GFG
{
    /* The function returns index
    of the first non-repeating
    character in a string. If
    all characters are repeating
    then returns INT_MAX */
    static int firstNonRepeating(String str)
    {
        int NO_OF_CHARS = 256;

        // Initialize all characters
        // as absent.
        int []arr = new int[NO_OF_CHARS];
        for (int i = 0; i < NO_OF_CHARS; i++)
            arr[i] = -1;

        // After below loop, the
        // value of arr[x] is going
        // to be index of x if x
        // appears only once. Else
        // the value is going to be
        // either -1 or -2.
        for (int i = 0; i < str.Length; i++)
        {
            if (arr[str[i]] == -1)
                arr[str[i]] = i;
            else
                arr[str[i]] = -2;
        }

        int res = int.MaxValue;
        for (int i = 0; i < NO_OF_CHARS; i++)

            // If this character occurs
            // only once and appears before
            // the current result, then
            // update the result
            if (arr[i] >= 0)
                res = Math.Min(res, arr[i]);

        return res;
    }

    // Driver Code
    public static void Main()
    {
        String str = "geeksforgeeks";
        int index = firstNonRepeating(str);
        if (index == int.MaxValue)
            Console.Write("Either all characters are " +
                        "repeating or string is empty");
        else
            Console.Write("First non-repeating character"+
                                " is " + str[index]);
    }
}

// This code is contributed by Rajput-Ji

Javascript

<script>

// JavaScript program to find first
// non-repeating character
// using 1D array and one
// traversal.

    /* The function returns index
of the first non-repeating
character in a string. If
all characters are repeating
then returns INT_MAX */
    function firstNonRepeating(str)
    {
        let NO_OF_CHARS = 256;

    // Initialize all characters
    // as absent.
    let arr = new Array(NO_OF_CHARS);
    for (let i = 0;
             i < NO_OF_CHARS; i++)
        arr[i] = -1;

    // After below loop, the
    // value of arr[x] is going
    // to be index of x if x
    // appears only once. Else
    // the value is going to be
    // either -1 or -2.
    for (let i = 0;
             i < str.length; i++)
    {
        if (arr[str[i].charCodeAt(0)] == -1)
            arr[str[i].charCodeAt(0)] = i;
        else
            arr[str[i].charCodeAt(0)] = -2;
    }

    let res = Number.MAX_VALUE;
    for (let i = 0; i < NO_OF_CHARS; i++)

        // If this character occurs
        // only once and appears before
        // the current result, then
        // update the result
        if (arr[i] >= 0)
            res = Math.min(res, arr[i]);

    return res;
    }

    // Driver Code
    let str = "geeksforgeeks";
    let index = firstNonRepeating(str);
    if (index == Number.MAX_VALUE)
        document.write("Either all characters are " +
                       "repeating or string is empty");
    else
        document.write("First non-repeating character"+
                             " is " + str[index]);


// This code is contributed by patel2127

</script>

Output

First non-repeating character is f

Time Complexity: O(n)
Auxiliary Space: O(1)

Alternate Implementation:

This is coded using a HashMap or Hashing Technique.

If the element(or key) repeats in the string the HashMap (or Dictionary) will change the value of that key to None.

This way we will later on only be finding keys whose value is “not None”.

Implementation:

C++

// C++ implementation of
// above approach
#include <bits/stdc++.h>
#include <unordered_map>
using namespace std;

// Function to make a frequency
// dictionary of characters in a string.
unordered_map<char, int> makeHashMap(string string)
{
    // Create an empty HashMap
    unordered_map<char, int> d1;

    // To store each character and its frequency
    char c;

    // Traversing through the string
    for (int i = 0; i < string.length(); i++) {

        // Extracting each character
        c = string[i];

        // If the character has occurred before
        if (d1.find(c) != d1.end()) {

            // Increment its frequency
            d1++;
        }

        // If the character has not occurred before
        else
            d1 = 1;
    }

    // Update value of each key such that
    // if frequency of  frequency of  a key
    // is equal to 1, then it is set to 0,
    // else set the value equal to None
    for (auto it = d1.begin(); it != d1.end(); it++)
    {
        if (it->second == 1)
            it->second = 0;
        else
            it->second = -1;
    }

    return d1;
}

// Function to return the first
// non repeating character
char firstNotRepeatingCharacter(string s)
{
    // Make a frequency dictionary
    unordered_map<char, int> d = makeHashMap(s);

    // Variable to store the result
    char nonRep = '.';

    // Traversing through the string only once
    for (int i = 0; i < s.length(); i++) {
        char c = s[i];

        // If the character's value in the
        // frequency dictionary is "not None"
        if (d == 0) {

            // Store it in the result
            nonRep = c;

            // Break out of the loop
            break;
        }
    }

    // If no non-repeating character is found
    if (nonRep == '.')
        return '_';
    else
        return nonRep;
}

// Driver Code
int main()
{
    string s = "bbcdcca";
    cout << firstNotRepeatingCharacter(s);
    return 0;
}

Java

// Java implementation of
// above approach
import java.util.HashMap;

class GFG {

    // Function to make a frequency
    // dictionary of characters in a string.
    public static HashMap<Character, Integer> makeHashMap(String string)
    {
        // Create an empty HashMap
        HashMap<Character, Integer> d1 =
                    new HashMap<Character, Integer>();

        // To store each character and its frequency
        char c;

        // Traversing through the string
        for (int i = 0; i < string.length(); i++) {

            // Extracting each character
            c = string.charAt(i);

            // If the character has occurred before
            if (d1.containsKey(c)) {

                // Increment its frequency
                Integer oldValue = d1.get(c);
                d1.put(c, oldValue + 1);
            }

            // If the character has not occurred before
            else
                d1.put(c, 1);
        }

        // Update value of each key such that
        // if frequency of  frequency of  a key
        // is equal to 1, then it is set to 0,
        // else set the value equal to None
        for (Character key : d1.keySet()) {
            if (d1.get(key) == 1)
                d1.put(key, 0);
            else
                d1.put(key, null);
        }

        return d1;
    }

    // Function to return the first
    // non repeating character
    public static Character firstNotRepeatingCharacter(String s)
    {
        // Make a frequency dictionary
        HashMap<Character, Integer> d = makeHashMap(s);

        // Variable to store the result
        Character nonRep = null;

        // Traversing through the string only once
        for (int i = 0; i < s.length(); i++) {
            Character c = s.charAt(i);

            // If the character's value in the
            // frequency dictionary is "not None"
            if (d.get(c) != null) {

                // Store it in the result
                nonRep = c;

                // Break out of the loop
                break;
            }
        }

        // If no non-repeating character is found
        if (nonRep == null)
            return '_';
        else
            return nonRep;
    }

    // Driver Code
    public static void main(String args[])
    {
        String s = "bbcdcca";
        System.out.println(firstNotRepeatingCharacter(s));
    }
}

Python3

# Python implementation of
# above approach
from collections import Counter


def makeHashMap(string):

    # Use Counter to make a frequency
    # dictionary of characters in a string.

    d1 = Counter(string)

    # Update value of each key such that
    # if frequency of  frequency of  a key
    # is equal to 1, then it is set to 0,
    # else set the value equal to None
    d1 = {(key): (0 if d1[key] == 1 else None)
          for key, value in d1.items()}

    return d1


def firstNotRepeatingCharacter(s):

    d = makeHashMap(s)

    # variable to store the first
    # non repeating character.
    nonRep = None

    # Traversing through the string only once.
    for i in s:
        if d[i] is not None:

            '''
            As soon as the first character in the string is
            found whose value in the HashMap is "not None",
            that is our first non-repeating character.
            So we store it in nonRep and break out of the
            loop. Thus saving on time.
            '''
            nonRep = i
            break

    if nonRep is not None:
        return nonRep
    else:

        # If there are no non-repeating characters
        # then "_" is returned
        return str("_")


# Driver Code
print(firstNotRepeatingCharacter('bbcdcca'))

# This code is contributed by Vivek Nigam (@viveknigam300)

C#

using System;
using System.Collections.Generic;

public class Program {
    // Function to create a Dictionary with characters and
    // their frequency
    public static Dictionary<char, int>
    MakeHashMap(string s)
    {
        // Initializing an empty dictionary
        var d1 = new Dictionary<char, int>();
        // Iterating through each character in the string
        foreach(char c in s)
        {
            // If the character is already in the
            // dictionary, increment its frequency
            if (d1.ContainsKey(c)) {
                d1++;
            }
            // Else add the character with frequency 1 to
            // the dictionary
            else {
                d1 = 1;
            }
        }

        // Initializing another empty dictionary for marking
        // repeated characters
        var d2 = new Dictionary<char, int>();

        // Iterating through each key-value pair in the
        // first dictionary
        foreach(var kvp in d1)
        {
            // If the character occurs only once, set its
            // value to 0 in the second dictionary
            if (kvp.Value == 1) {
                d2[kvp.Key] = 0;
            }
            // Else set its value to -1 in the second
            // dictionary
            else {
                d2[kvp.Key] = -1;
            }
        }

        // Return the second dictionary
        return d2;
    }

    // Function to find the first non-repeating character in
    // the given string
    public static char FirstNotRepeatingCharacter(string s)
    {
        // Create a dictionary with the frequency of each
        // character
        var d = MakeHashMap(s);

        // Variable to store the first non-repeating
        // character
        char nonRep = '.';

        // Iterate through each character in the string
        foreach(char c in s)
        {
            // If the frequency of the character is 0, it is
            // non-repeating
            if (d == 0) {
                // Store the non-repeating character and
                // break out of the loop
                nonRep = c;
                break;
            }
        }

        // If no non-repeating character is found, return
        // '_'
        if (nonRep == '.') {
            return '_';
        }
        else {
            // Else return the first non-repeating character
            return nonRep;
        }
    }

    // Main method
    public static void Main()
    {
        string s = "bbcdcca";
        Console.WriteLine(FirstNotRepeatingCharacter(s));
    }
}

Javascript

// Javascript implementation of
// above approach
function makeHashMap(string) {

    // Use Map to make a frequency
    // dictionary of characters in a string.
    let d1 = new Map();
    for (let i = 0; i < string.length; i++) {
        let char = string.charAt(i);
        if (d1.has(char)) {
            d1.set(char, d1.get(char) + 1);
        } else {
            d1.set(char, 1);
        }
    }

    // Update value of each key such that
    // if frequency of a key is equal to 1, then it is set to 0,
    // else set the value equal to null
    d1.forEach(function(value, key) {
        if (value === 1) {
            d1.set(key, 0);
        } else {
            d1.set(key, null);
        }
    });

    return d1;
}

function firstNotRepeatingCharacter(s) {
    let d = makeHashMap(s);

    // variable to store the first
    // non repeating character.
    let nonRep = null;

    // Traversing through the string only once.
    for (let i = 0; i < s.length; i++) {
        let char = s.charAt(i);
        if (d.get(char) !== null) {
            /*
            As soon as the first character in the string is
            found whose value in the HashMap is "not null",
            that is our first non-repeating character.
            So we store it in nonRep and break out of the
            loop. Thus saving on time.
            */
            nonRep = char;
            break;
        }
    }

    if (nonRep !== null) {
        return nonRep;
    } else {
        // If there are no non-repeating characters
        // then "_" is returned
        return "_";
    }
}

// Driver code
console.log(firstNotRepeatingCharacter("bbcdcca"));

// This code is contributed by Prajwal Kandekar

Output

d

Time Complexity: O(N)
Auxiliary Space: O(N)

Method 3:  Another simple approach to this problem without using any hashmap or array is mentioned below. We can find the first non-repeating character by just using single for loop.

Another Approach: 

To count the frequency of character we can do the following step:

frequency of a character = length_of_string – length_of_string_without_that _character

for example: Given String is “helloh” and we want to count frequency of character “h” so by using the above formula we can say

frequency of character “h” = 6(length of string) – 4(length of string without “h”)  = 2

So by this way, we can count the frequency of every character in a string and then if we found count == 1 that means that character is the first non-repeating character in the string.

Implementation: Implementation of the above method in java is shown below:

C++

#include<bits/stdc++.h>
using namespace std;

// C++ implementation of above approach

void firstNonRepeating(string s)
{
    bool flag = false;
    int index = -1;
    for (int i = 0; i < s.size(); i++) {

        // Here I am replacing a character with "" and
        // then finding the length after replacing and
        // then subtracting  length of that replaced
        // string from the total length of the original
        // string
        string temp = s;
        temp.erase(remove(temp.begin(), temp.end(), s[i]), temp.end());

        int count_occurrence = s.size() - temp.size();

        if (count_occurrence == 1)
        {
            flag = true;
            index = i;
            break;
        }
    }

    if (flag)
        cout << "First non repeating character is " << s[index] << endl;

    else
        cout << "There is no non-repeating character is present in the string" << endl;
}



int main(){

    // Driver Code
    string s = "GeeksforGeeks";
    firstNonRepeating(s);

    return 0;
}

// The code is contributed by Nidhi goel.

Java

// Java program for the above approach
public class first_non_repeating {


    static void firstNonRepeating(String s)
    {
        boolean flag = false;

        int index = -1;

        for (int i = 0; i < s.length(); i++) {

            // Here I am replacing a character with "" and
            // then finding the length after replacing and
            // then subtracting  length of that replaced
            // string from the total length of the original
            // string
            int count_occurrence
                = s.length()
                  - s.replace(
                         Character.toString(s.charAt(i)),
                         "")
                        .length();

            if (count_occurrence == 1) {

                flag = true;

                index = i;

                break;
            }
        }

        if (flag)
            System.out.println(
                "First non repeating character is "
                + s.charAt(index));

        else
            System.out.println(
                "There is no non-repeating character is present in the string");
    }

    // Driver Code
    public static void main(String arg[])
    {

        String s = "GeeksforGeeks";

        firstNonRepeating(s);
    }
}
// This Solution is contributed by Sourabh Sharma.

Python3

# Python implementation of above approach
def firstNonRepeating(s):

    flag = False
    index = -1
    for i in range(len(s)):

        # Here I am replacing a character with "" and
        # then finding the length after replacing and
        # then subtracting  length of that replaced
        # string from the total length of the original
        # string
        count_occurrence = len(s) - len(s.replace(s[i],''))

        if (count_occurrence == 1):
            flag = True
            index = i
            break

    if (flag):
        print(f"First non repeating character is {s[index]}")

    else:
        print("There is no non-repeating character is present in the string")

# Driver Code
s = "GeeksforGeeks"
firstNonRepeating(s)

# This code is contributed by shinjanpatra

C#

// C# program for the above approach
using System;

class first_non_repeating {

    static void firstNonRepeating(string s)
    {
        bool flag = false;

        int index = -1;

        for (int i = 0; i < s.Length; i++) {

            // Here I am replacing a character with "" and
            // then finding the length after replacing and
            // then subtracting  length of that replaced
            // string from the total length of the original
            // string
            int count_occurrence
                = s.Length
                  - s.Replace(Char.ToString(s[i]), "")
                        .Length;

            if (count_occurrence == 1) {

                flag = true;

                index = i;

                break;
            }
        }

        if (flag) {
            Console.WriteLine(
                "First non repeating character is "
                + s[index]);
        }

        else {
            Console.WriteLine(
                "There is no non-repeating character is present in the string");
        }
    }

    // Driver Code
    public static void Main()
    {

        string s = "GeeksforGeeks";

        firstNonRepeating(s);
    }
}
// This Solution is contributed by Samim Hossain Mondal.

Javascript

<script>

// JavaScript implementation of above approach
function firstNonRepeating(s)
{
    let flag = false;
    let index = -1;
    for (let i = 0; i < s.length; i++) {

        // Here I am replacing a character with "" and
        // then finding the length after replacing and
        // then subtracting  length of that replaced
        // string from the total length of the original
        // string
        let count_occurrence
            = s.length - s.replaceAll(s[i],'').length;

        if (count_occurrence == 1)
        {
            flag = true;
            index = i;
            break;
        }
    }

    if (flag)
        document.write(
            "First non repeating character is "
            + s[index],"</br>");

    else
        document.write(
            "There is no non-repeating character is present in the string","</br>");
}

// Driver Code
let s = "GeeksforGeeks"
firstNonRepeating(s)

// This code is contributed by shinjanpatra

</script>

Output

First non repeating character is f

Time Complexity: O(N)
Auxiliary Space: O(1)